In the espionage field, it is common practice to employ eavesdropping devices which are capable of responding to sound waves within a room, modulating a carrier with the sound waves, and transmitting a modulated RF signal which can be received at a location spaced from the room and monitored. Such eavesdropping devices are commonly quite small so that they can be easily concealed in a room and, generally, produce a relatively weak transmitted electromagnetic energy signal, the power level and, therefore, the range of transmission being determined by the proximity of the monitoring receiver. The power level of the transmitted signal can be as low as, for exampled, 500 microwatts. In addition, such transmitters are normally battery powered and can be remotely activated so that they remain dormant most of the time and are activated only when an operator at the monitoring location wishes to listen to events occurring within the room containing the device.
Except for the possibility of a physical search, it is difficult to determine the presence of such an eavesdropping device. While a physical search can be conducted, such a search is difficult and time consuming and may not reveal a transmitter which has been embedded in a wall or in some piece of furniture or appliance within the room and which has been permitted to remain dormant for an extended interval. Thus, it is desirable to be able to periodically or continuously investigate a room to determine the presence of operating eavesdropping devices without the necessity of conducting a physical search of the location upon each such occasion.
One phenomenon which can be employed, and which has been recognized as being usable for this purpose, is the regeneration phenomenon. It is well known that if the signal transmitted by the eavesdroppong device is received and demodulated and if the demodulated signal is then propagated in the form of acoustic energy within the room, the eavesdropping device will receive that acoustic energy, along with other sound in the room, and retransmit a signal modulated therewith, forming a regenerative feedback loop which results in a characteristic tone, most often a squeal, which reveals the presence of the device whereupon it can be located and removed. An example of a system of this type is shown in U.S. Pat. No. 3,473,127, Williams et al.
However, there are certain difficulties inherent in the use of this approach. One such difficulty is that the frequency of the carrier used by the eavesdropping transmitter is unknown. While, as a practical matter, the general range of frequencies over a rather broad spectrum can be determined, it is necessary to use a rather broad-band detector and broad-band amplifiers, or else to use the technique of sweeping through a wide spectrum of frequencies, hoping that the frequency used by the eavesdropping device will be encountered for a sufficiently long time to establish the regenerative feedback loop. The "spectrum sweeping" approach has been found to have substantial shortcomings in view of the very low power of the eavesdropping transmitter and the need to tune the sweeping device to the transmitter frequency for some finite interval in order to establish the loop. The broad-band detector and amplifier approach is, therefore, somewhat more desirable, but has a further disadvantage. This disadvantage has to do with the existence of radio or other noise signals from various sources, which interfering signals may be of a much greater power level than the signal produced by the eavesdropping transmitter. Indeed, the signal from a relatively close broadcast transmitter can exceed the eavesdropping transmitter power by many multiples. Thus, any broad-band detector and amplifier having sufficient gain to reproduce the eavesdropping transmitter's signal will saturate and become useless in the presence of such interfering signals, rendering the system inoperative.
It will further be recognized that the system must operate quickly so that an operator at the monitoring station will not become aware of efforts to detect the eavesdropping transmitter, giving him an opportunity to deactivate it.